1. Field of the Invention
This invention relates to a position detecting device of an electromagnetic induction system and a position input device including this position detecting device.
2. Description of Related Art
As disclosed in e.g. Japanese Patent Laid-Open No. 2006-309308, a position input device composed of a pen-type position indicator and a position detecting device is known. The position detecting device includes a sensor unit for detecting the position indicated by the position indicator.
This sensor unit generally includes a sensor substrate of a flat plate shape and an indication detecting plane to detect the input position indicated by the position indicator. In the case of a position detecting device of an electromagnetic induction system, on the sensor substrate, a large number of long thin loop coils are so provided as to be arranged in each of the X-axis direction and the Y-axis direction. The position indicator includes a resonant circuit composed of a coil and a capacitor.
Furthermore, the position detecting device makes a current of a specific frequency (i.e., a transmission current for excitation) flow through the respective loop coils of the sensor unit, one coil by one coil, to generate a magnetic field from the loop coils. When the position indicator is close to the loop coil generating the magnetic field, the resonant circuit of the position indicator resonates due to electromagnetic induction to generate an induced magnetic field. Next, the generation of the magnetic field from the loop coil is stopped. Subsequently, the induced magnetic field generated from the resonant circuit of the position indicator is received by the loop coil and a signal current (i.e., a reception current) flowing through the loop coil is detected. The position detecting device carries out this operation for each one of the loop coils and detects the position of the position indicator based on the reception current.
Directly beneath the sensor substrate, on which a large number of loop coils are formed, a magnetic path sheet (i.e., a yoke sheet) to stabilize the magnetic characteristics of the loop coil is provided. If a conductor plate such as an earth substrate is provided directly beneath the sensor substrate, on which a large number of loop coils are formed, an eddy current flows through the conductor plate due to the influence of an alternating magnetic field from the loop coil, and this eddy current adversely affects the operation of the device. Therefore, the magnetic path sheet (the yoke sheet) is so configured as to work also as a shield member (refer to, e.g., Japanese Patent Laid-Open No. 2007-157107).